Such motors, preferably employing electronic commutation, are used, because of their low axial moment of inertia, for jobs where an electric motor's RPM must very quickly respond to electrical commands, e.g. for fast positioning of parts, or servo-assistance to movements. For this purpose, one desires such a motor to have a very uniform torque. One generally achieves this by a three-phase configuration of the motor, in which each of the phases has an essentially sinusoidal current applied to it, and the motor is so designed that, in the phases or strands of the multi-phase stator winding, sinusoidal voltages are induced. One also calls such a motor a “sine motor.”
In such motors, the phenomenon occurs that the boundaries between the individual rotor poles, the so-called “pole boundaries,” seek the positions of the largest air gaps. For the observer, this has the appearance as if the pole boundaries were attracted by the slots of the stator. This effect is called “cogging.” The torque created thereby is called “cogging torque” because it seeks to hold the rotor in particular rotational positions.
This effect is generated by a so-called “reluctance torque,” i.e. during the rotation of the rotor, relative to the stator, magnetic energy is stored in the magnetic circuit of the motor in certain rotation angle ranges, and, in other rotation angle ranges, this magnetic energy is released, analogous to when one alternately tensions a spring and releases it. For the storing, energy must be supplied to the rotor, i.e. the rotor is being braked thereby, and conversely, where the stored energy is being released, the rotor is being driven. If one turns the rotor of such a motor by hand, one has the impression that one “feels every slot.”
In the context of many drive applications, this reluctance torque is disruptive, so that there one is forced to use so-called “ironless” stator windings in which no reluctance torque arises. However, the power of such motors with ironless stators is generally insufficient because their air gap is very large. This leads to a high “specific weight” (weight/power ratio), i.e. the relationship of motor power to motor volume or motor power to motor weight is unfavorable with them.
Some have tried to overcome this problem by giving the pole shoes of the rotor a particular form, but that has led, thus far, to a structure in which the specific weight was unfavorable.